Handle twap in orderbook

[anxolin]

This PR implements and test the logic in TWAP that instructs the watch-towers what is the next polling time for the case, the current part is already in the order book.

This is very important for the scalability of watch towers, since otherwise they would need to check on every block. This PR will allow a DCA that executes once a month, to be polled only once a month (two at most, since first polling returns the current part, and next polling for the same period will instruct to wait for approximately one month )

The test will cover various edge cases in the execution of a 10 parts TWAP order. Read the tests for more details. As a summary:

• Will verify that when is the time of part N, it instructs watch tower to wait until N+1 starts
• Will verify that if we are in the last part, then it will instructs watch tower to not poll again this order
• It also tests some edge-cases that should not happen, but formalises the assumption of the responsibility of this function. This function is supposed to be called only if the current part is yielding an order that is already in the orderboook, so it shouldn’t be possible that the TWAP is expired or that is hasn’t started. However, the implementation handles these cases, and the test also covers them

Test

yarn test

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[coveralls]

coverage: 79.121% (+1.8%) from 77.295% when pulling f6e5f5a800a80ea97015974804b167394fdb44bb on handle-twap-in-orderbook into a026ae18d4918d72744876aab767e258ac8b7b72 on main.